The embryos, dating back 190 million years, are of Massospondylus carinatus dinosaurs. Related to the giant sauropods (long-necked, plant-eating dinosaurs), the animals grew to be 16 feet (5 meters) long and were the most common dinosaur in South Africa, where the embryos were found.

"As the animal grew from embryo to adult it went from an awkward-looking, big-headed quadruped to a small-headed, long-necked adult that was quite comfortable running around on its hind legs," said Robert Reisz, a vertebrate paleontologist at the University of Toronto at Mississauga, Canada.

The discovery is unique because it is the first time that scientists have been able to chart a dinosaur's growth from embryo to adulthood.

The findings, which are published tomorrow in the journal Science, may provide clues about how the giant sauropod dinosaurs evolved. The research was supported in part by the National Geographic Society's Committee for Research and Exploration.

Complete Skeleton

The embryos were found during a road construction project in South Africa in 1978. But researchers have not attempted to expose them from the fossil eggshells and surrounding rock until now.

"In general, dinosaur eggs are relatively rare, but embryos are even rarer," Reisz said. "This [find] is unique not only because it is the oldest preserved embryo but because the preservation is fantastic."

Reisz's research assistant, Diane Scott, worked on the delicate, 2.4-inch-long (6-centimeter-long) eggs for more than a year. She uncovered two embryos, including a full, 6-inch-long (15-centimeter-long) skeleton curled up inside its egg.

At first, the scientists were not able to identify the animal. But as the work continued they recognized it as a Massospondylus. The species belongs to a group of plant-eaters known as prosauropods, a smaller relative of the giant sauropods.

Since scientists already have numerous juvenile and adult skeletons of the Massospondylus, the newly uncovered embryos enabled researchers to study how these dinosaurs grew from embryos into adults.